1. Field of the Invention
This invention pertains to mechanical and chemical operations in wells. In one embodiment, polymer and method of preparing viscous fluid for use in wells is provided. In another embodiment, material and methods are provided for coating of sand control screens used in wellbores. In another embodiment, this invention pertains to delayed mechanical or flow changes in a wellbore after equipment is placed in the well. In another embodiment, this invention pertains to equipment that is placed in a wellbore and is degraded in the wellbore by contact with aqueous fluid.
2. Discussion of Related Art
A large number of mechanical and chemical operations are carried out in and around wells. Most of these wells are used for producing hydrocarbons from the earth. They are located at depths ranging from a few hundred feet below the surface of the earth to more than 30,000 feet. The temperature at the bottom of the wells likewise varies over a wide range—from about 100° F. to more than 400° F.
After the hole is drilled in the earth in the process of constructing a well, the process of placing casing in the well and cementing it in place is commenced. Mechanical devices to aid in the cementing process may be placed on the outside of the casing before it is placed in the hole. Instruments and communication cables may be placed on the casing. Multiple lateral holes may be drilled from a single hole and casing may be placed in each. When casing has been cemented, the process of “completing” the well may begin. This involves forming holes (“perforating”) the casing opposite an interval of a formation where fluids are to be produced or injected and, in most cases, placing tubing in the well. Various types of mechanical equipment may be placed in the wellbore, for safety, flow control and other purposes. Viscous, non-damaging fluids having a selected specific gravity are needed in wells during completion operations. In many wells various types of treatment fluids are then injected into the well to provide greater capacity of the well to produce hydrocarbons, in processes such as hydraulic fracturing and acidizing, called “stimulation” processes. The use of a degradable polymer in the form of ball sealers or particulates to divert fluid or control fluid loss from a well during completion or stimulation operations has been disclosed. (U.S. Pat. No. 4,716,964)
In some wells, the formation where hydrocarbons are found has low mechanical strength, which can result in “sand” being produced into the well along with hydrocarbons. The well then requires application of a “sand control” process. One of these processes requires placing a “screen” in the well. The solid particles (cuttings) and drilling fluid in the well may plug or partially plug the screen as it is placed in a well. This problem can be particularly severe in directional or horizontal wells. A recent U.S. Patent Application Publication (US2002/0142919 A1) discloses screen coatings that melt or dissolve within a wellbore and release reactive materials effective in degrading or dissolving materials that could plug a screen. The problem of screen plugging during placement was recognized many years ago (“Downhole Protection of Sand Control Screens,” Society of Petroleum Engineers Paper No. 8803, 1980).
In well operations used for completing or stimulating a well, viscous fluids may be used. In most cases, it is desirable that the fluid become lower viscosity with time after it is placed in a well or formation around a well. When the fluid becomes low viscosity it should contain no significant amount of solid or gel material. One example application of such fluids is hydraulic fracturing of wells. U.S. Patent Application Publication 2003/0060374A1, which is hereby incorporated by reference herein, discloses the use of highly concentrated degradable polymers in an aqueous liquid in such application. As explained in that Publication, there is a need for fracturing fluids that degrade to low viscosity without leaving a residue.
Other applications where a viscous fluid may be injected into a well or used in a well include completion fluids, perforating fluids and fluids for carrying gravel (sand) into a well. These fluids are preferably solids-free and degradable to low viscosity fluid having low solid or gel content that could degrade permeability of a porous rock. Other applications where a viscous liquid in a wellbore may be advantageous include a completion or workover fluid that is placed in a well during running of a mechanical device into the well or other mechanical operation in the well. These fluids may contain high concentrations of compounds soluble in water that increase the density of the fluid, such as sodium bromide or zinc bromide, or solid weighting materials. The viscosifying material in the fluids should degrade with time and leave little or no residue of solid or gel that could damage the permeability of a formation around the well.
A wide variety of mechanical devices are placed in wells during completion and workover operations. These devices are used to control fluid flow, to seal around tubulars in the well, to perform measurements of physical or chemical parameters and various other purposes. These devices may be needed for only a limited time and then an operator may wish to have them no longer effective or to no longer have mechanical strength. For example, packers, bridge plugs and cement retainers may be needed for a limited time in a well. There may be a need to release a mechanical device or open a port after a selected time in an inaccessible portion of a wellbore, such as in an annulus between tubular strings, where an aqueous fluid is located.
What are needed in a variety of well operations or processes are viscous liquids that degrade to low viscosity liquid at a predictable rate and leave low amounts of solid or gel residue, a degradable coating for screens, and solids that lose mechanical strength at a predictable rate in the presence of an aqueous liquid to allow delayed flow or mechanical changes in inaccessible locations in wellbores or degradation of mechanical equipment that is no longer needed in a wellbore.